The aim of this investigation is to provide a comparative biochemical study, at the molecular level, of the oxygen-carrying pigments. Enormous efforts have been applied to hemoglobin in many laboratories. We are attempting to obtain complementary information for the non-heme oxygen carriers, hemerythrin and hemocyanin, in the hope of elucidating relationships between molecular structure and biological function, in this case, of oxygen binding. Our new investigations will focus on two objectives: (l) to establish further details of the structure of the active site; (2) to examine the effects of chemical and genetic modifications of tertiary and quarternary structure on oxygen uptake and heterotropic interactions. As the progress report indicates the conjunction of a variety of methods has established that the two iron atoms in oxyhemerythrin act in concert, that they are Fe(III) in oxidation state and antiferromagnetically coupled through an Fe(III)-O-Fe(III) bridge, and that they are linked to the protein backbone through four identified His and two identified Tyr residues. Using resonance Raman and Fourier transform infrared spectroscopy, we shall now attempt to determine the precise steric position of each of the bound O2 atoms. With additional help from high resolution NMR, we shall try to find which ligands in deoxyhemerythrin occupy the coordination sites vacated by O2 and O ions. Using chemical, genetic and NMR methods we will try to delineate which residues suggested from current low-resolution X-ray diffraction play crucial roles in determining the functional behavior of hemerythrin. Similar studies will be carried out with hemocyanin.